1. Field of the Invention
The present invention relates to a pneumatic tire reinforced with steel cords.
Pneumatic tires reinforced with steel cords have been used preferably for high speed and heavy load vehicles for transportation as well as construction vehicles. But due to the heavy specific gravity of steel cords, the weight of tires becomes heavy so that a large amount of fuel has been consumed.
2. Prior Art
In order to reduce the tire weight, it has been attempted to use a specially compounded rubber or to reduce the thickness of rubber in some portions, to the extent that reduced faults occur in the tire functions. But the amount of steel cords has not been reduced because if the amount of steel cords is reduced, the safety ratio of tires is lowered.
Besides, in a pneumatic radical tire reinforced with steel cords, concentration of stress usually occurs at the interface between rigid steel cords and flexible rubber, thereby readily causing rubber separation from the end portions of steel cords or rubber separation in belt plies in a radial tire. Moreover, a conventional pneumatic tire reinforced with steel cords caused a separation failure due to detachment of a surface metal plating induced by water penetration into an interface between an iron body and a surface metal plating of steel cords when being exposed to high temperature and high humidity for a long time.
The inventors have conducted various investigations and experiments in order to reduce the amount of steel cords in order to lighten tire weight and thereby decreasing fuel consumption and diminishing separation failures in a pneumatic tire. As a result, in Japanese Patent Application No. 55-125625 laid open on Mar. 26, 1982 under No. 57-51502 the inventors proposed a pneumatic tire reinforced at least partially with steel cords in which most of steel filaments of steel cords comprise an iron material containing carbon at 0.75% to 0.85% by weight and the steel cords have the tensile strength calculated by the following formulas; ##EQU2## In the above formulas, D is the diameter (mm) of steel filaments of a steel cord. N is the number of steel filaments of a steel cord. W is the weight (g) of a steel cord per 1 m. TS is tensile strength (kg) of a steel cord. The denominator, 7.86 shows the specific gravity of iron. The above formula (1) and (2) are applied in the range of N being 3 to 50 and D being 0.15 to 0.40. If D is less than 0.15 mm, productivity is poor and production is not practical because of high cost. If D is more than 0.40 mm, the rigidity of a steel cord becomes too high so that the tire easily undergoes separation, and fatigue resistance is reduced. In respect to N, when the diameters of the core filaments are different from each other and outer filaments, the number, m, of core filaments shall be taken as 1 to be added to the number, n, of outer filaments. Namely, N is taken as n+1.
On the other hand, the tensile strength of conventional steel cords used for tires can be calculated by the formula; ##EQU3##
In order to obtain a steel cord having the tensile strength satisfying TS value according to the above formula 1, the carbon content in an iron material for steel filaments should be in the range of 0.75 wt% to 0.85 wt% which is more than the carbon content 0.69 wt% to 0.73 wt% in conventional steel filaments. In case of a carbon content less than 0.75 wt%, the tensile strength is low. A carbon content more than 0.85 wt% makes heat treatment of the steel filaments very difficult and results in a deficiency in tenacity of the steel cord obtained. As a result, when a pneumatic tire employing such steel cords treads on projections and the like, they are readily broken down.
The steel filaments containing the above mentioned amount of carbon are brass-plated in order to increase the adhesiveness thereof to rubber. The preferable composition ratio of the brass is 60 wt% to 70 wt% copper and 30 wt% to 40 wt% zinc. If copper contained in brass is less than 60 wt%, the brass has a .beta. phase so that such brass-plated irons are impossible to be drawn into steel filaments in cooperation with the high hardness of iron materials. If copper contained in brass is more than 70 wt%, pin holes are formed on the brass when steel filaments are drawn. One of the special findings in said invention is that no breakage occurs between the steel filaments and the brass when a tire is exposed to high temperature and high humidity, by virtue of firm bonding between steel filaments having high carbon content and plated brass. This is because higher pressure is applicable to materials in a wet-drawing process using a lubricant than in a drawing process for conventional materials containing low carbon, so that brass is pressed on and bonded to iron cores when the materials pass through drawing dies. Assuming that the strength of tires is uniform, the reinforcing effect of a steel cord is small if the tensile strength of a steel cord is smaller than the value calculated from said formulae 1 and 2. Accordingly, the amount of steel cords can not be reduced. As a result, tire weight and fuel consumption are not so different from those of tires employing conventional steel cords. If on the contrary the tensile strength of a steel cord is bigger than the value calculated from said formula 1 and 2, the amount of steel cords corresponding to the increased amount of the tensile strength can be reduced in order to maintain the same strength as that of a tire comprising conventional steel cords. But the rigidity of a tire and the rigidity of the composite of rubber and steel cords become small so that the wear resistance of a tire diminishes.
The above described pneumatic tire provided by the senior Japanese patent application is lighter in weight and smaller in rolling resistance and fuel consumption than a conventional tire and also it enjoys superior moisture heat coverage. But a separation failure has been experienced at a belt portion when said tire treads on a projection like a nail. Said separation results from the deterioration of strength of belt portion due to accidental cutting of steel cords contained therein. Our investigation reveals that it is attributed to the fact that the knot strength of steel cords is lowered with the increase in tensile strength thereof.